[asterisk-users] Clarification on DAHDI Fax Detection

[Kevin P. Fleming]

On 01/03/2011 06:47 PM, Thomas Rymes wrote:
> On Jan 3, 2011, at 3:22 PM, Kevin P. Fleming wrote:
>
>> On 01/03/2011 11:26 AM, Tom Rymes wrote:
>
> [snip]
>
>>> 1.) Echo cancellation is automatically disabled upon recognition of a
>>> CNG tone, regardless of the faxdetect setting. This can only be disabled
>>> at compile time.
>>
>> No. CNG tone is never used to affect the state of an echo canceller. All G.168 compliant echo cancellers will respond to the CED tone (generated by the answering endpoint) and will reconfigure the echo canceller appropriately. Most modern ECs will *not* be disabled, but will enter a 'linear' mode where they can do some echo suppression but not complete cancellation. DAHDI will detect CED when most software echo cancellers are in use and will disable them (since none of the available software ECs can go into linear mode). The Digium HPEC software EC will detect CED on its own and enter linear mode.
>
> OK. Either way, though, the changes to echo cancellation are not affected by the faxdetect setting, right?

That is correct; the faxdetect setting and the echo canceller behavior 
are completely unrelated.

>
>>> 2.) faxdetect=incoming will, upon detection of a CNG tone, send the call
>>> to the fax extension.
>>
>> If the CNG tone arrives from the network side of the DAHDI channel (the far endpoint), then yes.
>
> Great. This is the typical usage, I presume, directing fax machines to FFA, Hylafax, another fax machine, or hangup (if this isn't a fax line).
>
> Is there a time limit to when DAHDI listens for faxes (say the first 10 seconds of a call?), or might it detect one in the middle of a ten minute call?

I haven't double-checked, but I believe the software DSP will be in 
place on the call until it sees a CNG tone, regardless of when that 
happens during the call.

>
>>> 3.) faxdetect=outgoing will ??????
>>
>> The same thing, but if the CNG tone is being sent towards the DAHDI channel (from the near endpoint). This is rarely used.
>
> [snip]
>
> I figured that must be it. Presumedly you might use this to perform some activity on an outgoing fax prior to sending it, such as logging something, etc? Maybe send it to FFA, receive it, and e-mail it to another server that faxes it out on a local number to save toll calls, etc?
>
> Thanks for the clarification, there's a lot of conflicting info out there.

Feel free to comment on wiki.asterisk.org if any of the information 
there led you astray; we'd like to get that to be the most accurate 
place for people to find this sort of information.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
skype: kpfleming | jabber: kfleming at digium.com
Check us out at www.digium.com & www.asterisk.org